Urea-formaldehyde (UF) resin adhesives are commonly used to produce wood composite products such as particleboard, medium-density fiberboard, and hardwood plywood panels. These UF resins are considered good binders in these applications due to high physical strength properties, fast curing times, and high cost-efficiency. Two major drawbacks to UF resin-based systems, however, are the limited strength durability of the resulting composite products as well as the emission of formaldehyde. Formaldehyde emissions are of particular concern when using UF resin-bonded boards for interior purposes such as sub-flooring, shelving, cabinets, and furniture. Air concentrations of formaldehyde above 0.1 parts per million (ppm) are associated with acute health effects, including watery eyes, burning sensations in the eyes, nose and throat, nausea, coughing, chest tightness, wheezing, skin rashes, headaches, fatigue, asthma, and other irritating effects. Formaldehyde has been shown to be cancer-causing in laboratory animals, although there is limited evidence of cancer-causing effects in humans. Nevertheless, it is classified as a “probable human carcinogen” by the United States Environmental Protection Agency (EPA) and the National Institute for Occupational Safety and Health.
Both the formaldehyde emission problem and the durability issues of UF resin-bonded wood products are linked to the underlying chemistry of the UF resin system. During synthesis of resin, hydroxymethyl groups are formed from the reaction of formaldehyde (F) and urea (U) as functional groups needed for the subsequent polymerization and curing processes. However, the reverse reaction of hydroxymethyl group formation also occurs during synthesis and subsequent curing processes to generate back some free formaldehyde, which is later emitted into the environment. The extent of the reverse reaction is generally proportional to the F/U mole ratio used in resin synthesis and is relatively small in comparison to the forward reaction, but still persists to the current low F/U molar ratio for resins of about 1.15 (Myers, G. E. Holzforschung 44:117-126 (1990); Forest Products Journal 34:35-41 (1984). This is the underlying mechanism for the formaldehyde emission phenomena of UF resin-bonded wood composite boards. This low F/U mole ratio of resin needed for lower emission, on the other hand, translates into a functionality value of about 2.3 formaldehyde molecules per urea molecule in current UF resins. Polymer molecular theory on the formaldehyde-based thermosetting resins indicates that the base monomer (for example, urea) needs to have a functionality of at least 3.0 or higher to make the resin polymers grow to a three-dimensional, fully cross-linked state (Flory, P. J. Polymer Chemistry, Cornell University Press, Ithaca, N.Y. (1953) p. 79.). Since the urea functionality in current UF resins is significantly lower than the theoretical value, a full cross-linking does not occur and the cured resin binders will result in limited strength durability of boards. The formaldehyde emission problem still persists at the current F/U mole ratio values of resin. Currently, UF resin formulation (mostly lowering of F/U mole ratio) and scavenger parameters have been pushed to limits for reduction of formaldehyde emission from boards, but significant further formaldehyde emission reductions are desired. From the above theoretical consideration, such a reduction in formaldehyde emission level for UF resin-bonded boards seems to require a significant redesigning of the starting molecule toward materials having higher functionality than urea. There accordingly remains a need in the art for interior-grade wood composite binder resins for improved starting materials as well as their formaldehyde condensation products that give superior resin properties of low cost, colorlessness, exceptionally good binding, and fast curing characteristics, as well as very low formaldehyde emissions. The present invention provides such advantages.